Dont raise ValidationError when there is no default settings object

[benjaoming]

Don't raise ValidationError when there is no default settings object, make current one the default.

Not sure yet if this change should go in, but opening the PR to flag for later.

[codecov-io]

Codecov Report

Merging #95 into master will decrease coverage by 0.04%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
- Coverage   76.21%   76.17%   -0.05%     
==========================================
  Files          26       26              
  Lines         555      554       -1     
==========================================
- Hits          423      422       -1     
  Misses        132      132

Impacted Files	Coverage Δ
django_nyt/models.py	84.55% <0%> (-0.13%)	:arrow_down:

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